1. Field of the Invention
The present invention concerns a re-entrant radio frequency (RF) cavity resonator filter which achieves a low level of generated passive inter-modulation (PIM) by having a tuning element which does not have contact with the fixed tuning post of the re-entrant cavity resonator.
2. Description of the Related Art
Conventional RF cavity resonators are typically tunable so that the resonator may be used across a given range of operating frequencies. Such a conventional RF resonator typically consists of a cavity body, either cylindrical or rectangular, in which a fixed tuning (resonator) post resides inside of the cavity body by being connected at one end to a wall of the cavity body. The fixed tuning post of the typical resonator is generally a tubular, cylindrical shape with a far end opposite the connection to the cavity body being open. The typical fixed tuning post is usually machined as an integral part of the cavity body, and the inside tube of the typical fixed tuning post accommodates a rotatable post that is also cylindrical in nature and that has a metallic tuning element attached at one end near the open end of the fixed tuning post. The metallic tuning element typically moves relative to the fixed tuning post by having screw-type threads on the outer surface of the metallic tuning element which correspond to screw-type threads provided on the inner surface of the inside tube of the fixed tuning post.
The end of the rotatable post which is opposite the metallic tuning element is usually attached to an adjusting knob which protrudes out of the cavity body near the cavity body wall to which the fixed tuning post is connected. Accordingly, the user of a typical RF cavity resonator can adjust the operating frequency of the resonator by turning the adjusting knob which turns the threaded metallic tuning post at the end of the rotatable post within the threads on the inside of the fixed tuning post, thereby linearly moving the metallic tuning element within the fixed tuning post in a controlled manner. As the metallic tuning element is moved relative to the open end of the fixed tuning post, the operating frequency of the cavity resonator is “tuned” to a new frequency.
Unfortunately, although the above-described typical RF cavity resonator achieves the goal of tuning the cavity resonator to a different frequency, there are certain deleterious effects caused by the use of the “screw-type” metallic tuning post mentioned above. Specifically, the metal-to-metal contact between the threaded outer surface of the metallic tuning element and the threaded inner surface of the fixed tuning post causes the generation of passive inter-modulation (PIM) during operation of the cavity resonator. The generated PIM can result in unwanted distortion of the outgoing signal from the cavity resonator.
In addition, repeated tuning of the typical RF cavity resonator can cause the threads on the outer surface of the metallic tuning element and the threads on the inner surface of the fixed tuning post to wear and deteriorate. When the threads become worn and deteriorated, they become mismatched and thereby result in unstable positioning of the metallic tuning element within the cavity body of the cavity resonator. This results in an inability to accurately move the metallic tuning to a specific location, thereby preventing accurate tuning of the cavity resonator in a repeatable and uniform fashion.
Accordingly, it is desirable to have a RF cavity resonator which reduces the amount of PIM generated by contact between the metallic tuning element and the inside of the fixed tuning post. In addition, it is desirable to have a cavity resonator which can be repeatedly tuned in an accurate, repeatable and uniform fashion.